This invention relates to a monitor for the automated non-invasive measurement of a patient's blood pressure comprising means for automatically inflating and deflating a cuff which is to be applied around a patient's extremity, in particular an arm, first and second pressure sensors coupled to the airway of that cuff, each sensor providing an electronic signal indicative of cuff pressure, and electronic means responding to the signals obtained by said pressure sensors for stopping the inflation of the cuff when a certain predetermined maximum cuff pressure is reached, for calculating the blood pressure, in particular from the pressure signal, and for detecting malfunctions of the pressure sensors.
Such a monitor is described in EP-Application No. 88 102 787.4 and thus is considered to represent the state of the art. However, other than the indication that two independent pressure sensors may be used to control each other in order to detect malfunctions, fact that the fact that sensors are required for monitoring the cuff pressure, in particular the maximum cuff pressure, and the fact that the monitor stops inflating the cuff when the maximum cuff pressure is achieved, the EP-88 102 787.4 does not describe any details of such a monitor.
In a monitor for the automated non-invasive measurement of a patient's blood pressure which includes a cuff as a means for exerting an external counter pressure upon the patient's artery, security means must be provided which prevent the surpassing of predetermined pressure values independently of the operation of blood pressure measurement. Such means must guarantee in case of failure of the electronic pressure sensor, of the inflating means and of the electronic means provided for evaluating the blood pressure, that a pressure can not be exerted which has too high a value or too long a duration and thus could injure the patient. If the same monitor should be appropriate for being used with newborns, children and adults, besides of the evaluation algorithms and, the threshold values of pressure sensors must be adapted to the patient. A proposition of the IEC determines the maximum cuff pressure for newborns at 165 mmHg, for children at 220 mmHg and for adults at 330 mmHg.
In presently available monitors, mechanical pressure switches are used which disconnect the supply voltage from at least relevant parts of the monitor when the maximum pressure is reached. Such a mechanical pressure switch is essentially comprised of an elastic membrane which is moved by the exerted pressure and actuates a micro switch if the exerted pressure surmounts a predetermined value. It suffers from being expensive, bulky, susceptible to shocks, and affected by wide tolerances. Further, since it has an elastic membrane, it is not free of wear. Finally, an essential drawback of such pressure switch consists in that its operation can only be tested when too high a pressure occurs, i.e. in case of a failure, but not during normal operation.
If provision has to be made that short pressure peaks in the system which may be caused by movement artifacts of the patient do not trigger the pressure switch, a certain damping must be provided in the tube system, which damping, however, has the disadvantage to extend the response time of the system.
In order to keep the expenditure low, in most cases only one pressure switch is used which has its threshold value set to the highest value, i.e. the value for adults. If the dependence of the pressure threshold on the patient has to be considered, either a pressure switch having three defined threshold values or three pressure switches having different threshold values are needed. The threshold value assigned to the patient must then be logically or mechanically enabled whereas the two other threshold values are to be disabled.